Negative electrode active material, nonaqueous electrolyte battery and battery pack

ABSTRACT

According to one embodiment, a negative electrode active material includes a compound having a crystal structure of monoclinic titanium dioxide. The compound has a highest intensity peak detected by an X-ray powder diffractometry using a Cu-Kα radiation source. The highest intensity peak is a peak of a (001) plane, (002) plane, or (003) plane. A half-width (2θ) of the highest intensity peak falls within a range of 0.5 degree to 4 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT Application No.PCT/JP2009/059080, filed May 15, 2009, the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a negative electrodeactive material, a nonaqueous electrolyte battery, and a battery pack.

BACKGROUND

Monoclinic titanium dioxide is disclosed in R. Marchand, L. Brohan, M.Tournoux, Material Research Bulletin 15, 1129 (1980). Further, JP-A2008-34368 (KOKAI) discloses a lithium ion secondary battery usingtitanium oxide TiO₂ having a bronze type structure. JP-A 2008-117625(KOKAI) discloses a lithium secondary battery using titanium dioxidehaving a crystal structure of bronze titanate type of which the highestintensity peak is a peak of a (110) plane.

WO2009/028553 A1 discloses a titanium oxide compound of which thehighest intensity peak is a peak of a (003) plane, and a half-width (2θ)of the highest intensity peak is 0.4 degree. Therefore, a reversiblecharge-discharge capacity is lowered.

A battery capacity of each of JP-A 2008-34368 (KOKAI), JP-A 2008-117625(KOKAI) and WO2009/028553 A1 is considerably lower than about 330 mAh/gwhich is a theoretical capacity in the case of using monoclinic titaniumdioxide for an active material. Further, a practical capacity of spineltype lithium titanate is 170 mAh/g, and, as compared to the spinel typelithium titanate, it is difficult to greatly improve the capacity byusing the titanium dioxide described in JP-A 2008-34368 (KOKAI) and JP-A2008-117625 (KOKAI), and WO2009/028553 A1 as the active material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a layered crystal structure ofTiO₂(B) according to a first embodiment;

FIG. 2 is a diagram schematically illustrating an aspect ratiocalculation method;

FIG. 3 is a sectional view showing a nonaqueous electrolyte batteryaccording to a second embodiment;

FIG. 4 is an enlarged sectional view showing a part A of FIG. 3;

FIG. 5 is an oblique view, partly broken away, schematically showinganother nonaqueous electrolyte battery according to the secondembodiment;

FIG. 6 is an enlarged sectional view showing a part B of FIG. 5;

FIG. 7 is an exploded perspective view showing a battery pack accordingto a third embodiment;

FIG. 8 is a block diagram showing an electric circuit of the batterypack of FIG. 7;

FIG. 9 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Example 1B;

FIG. 10 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Example 2B;

FIG. 11 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Example 3B;

FIG. 12 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Example 4;

FIG. 13 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Comparative Example 1;

FIG. 14 is a diagram showing an X-ray diffraction of a titanium dioxidepowder of Comparative Example 3; and

FIG. 15 is a diagram showing charge-discharge curves of Examples 1A and4 and Comparative Examples 3 and 4.

DETAILED DESCRIPTION

In general, according to one embodiment, a negative electrode activematerial includes a compound having a crystal structure of monoclinictitanium dioxide. The compound has a highest intensity peak which isdetected by X-ray powder diffractometry using a Cu-Kα radiation source.The highest intensity peak is a peak of a (001) plane, (002) plane, or(003) plane. A half-width (2θ) of the highest intensity peak fallswithin a range of 0.5 degree to 4 degrees. Hereinafter, the compound isreferred to as titanium oxide compound.

The embodiments will be described with reference to the drawings. Anidentical reference numeral is given to components which are common tothe embodiments, and an overlapping description is not repeated.Further, the diagrams are not more than those which are schematicallydrawn for the purpose of illustration and understanding of theembodiments. Shapes, dimensions, ratios, and the like in the diagramsmay partially differ from actual devices, and designs thereof canappropriately be changed by taking the following descriptions andwell-known technologies into consideration.

First Embodiment

As used herein, the crystal structure of monoclinic titanium dioxidemeans those represented by a space group C2/m and described in R.Marchand, L. Brohan, M. Tournoux, Material Research Bulletin 15, 1129(1980). Hereinafter, the crystal structure of monoclinic titaniumdioxide is referred to as TiO₂(B). The crystal structure of monoclinictitanium dioxide includes the case in which a lithium ion is containedin the crystal structure.

Shown in FIG. 1 is a diagram schematically showing a projection of the(001) plane in the layered crystal structure of TiO₂(B). An oxide ion Ais positioned at an apex of an octahedron shown in FIG. 1, and atitanium ion B is positioned at a central part of the octahedron.Skeletal structures X each of which is formed of the titanium ion B andthe oxide ion A are alternately disposed to form a tunneling structure.Further, a clearance Y in the skeletal structures X is a space servingas a host for a lithium ion. In the titanium oxide compound, a sitewhich is capable of absorption and release of the lithium ion can existon a crystal surface. Therefore, the titanium oxide compound has aproperty of being capable of intercalation (insertion) of the lithiumion into the clearance Y and allowing adsorption/release of the lithiumion on the crystal surface. Further, the compound is capable ofinsertion and adsorption of many foreign atoms, organic compounds, andthe like other than the lithium ion.

When Li⁺ is intercalated into the tunnel-like clearances which areobserved on the (001) plane, Ti⁴⁺ forming the skeleton is reduced toTi³⁺ to make it possible to maintain electrical neutrality of thecrystal. Therefore, since TiO₂(B) has one Ti⁴⁺ per unit lattice, it ispossible to newly insert one Li⁺ at the maximum between layers.Accordingly, a composition of the titanium oxide compound is representedby Li_(x)TiO₂ (provided that a value of x is changeable within a rangeof 0≦x≦1 by charge-discharge). Thus, the titanium oxide compound has atheoretical capacity of about 335 mAh/g which is about twice as much asthe known titanium oxide. As for a capacity per unit weight of atitanium compound, a spinel type lithium titanate such as Li₄Ti₅O₁₂ hasa theoretical capacity of about 175 mAh/g. Further, the number oflithium ions per one mole, which is capable of insertion/desorption ofthe spinel type lithium titanate Li₄Ti₅O₁₂, is three. Therefore, thenumber of lithium ions which is capable of insertion/desorption pertitanium ion is 3/5, and the theoretical maximum number of lithium ionswhich is capable of insertion/desorption per titanium ion is 0.6.

Symmetry of space groups in TiO₂(B) may be varied due to a distortioncaused by an intercalation amount and a type thereof. It is suggestedthat crystal grains are oriented toward the (001) plane in a compoundhaving TiO₂(B) crystal structure since the highest intensity peakdetected by the X-ray powder diffractometry using Cu-Kα radiation sourceof the compound is the peak of the (001) plane, (002) plane, or (003)plane. In the compound, since many (001) planes having the tunnelingstructure are exposed, it is considered that the movement of lithiumions is made smoother. As a result, it is possible to improve aneffective electrode capacity and a repetitive charge-dischargeperformance.

Further, the half-width (2θ) of the highest intensity peak is within arange of 0.5 degree to 4 degrees. The half-width of the peak includesinformation such as crystallinity and a crystallite size of the grainsand has a correlation with a performance of the negative electrodecontaining the titanium oxide compound. When the half-width of the peakis less than 0.5 degree, the performance of the negative electrode isundesirably deteriorated. The deterioration is caused by excessivedesorption of crystal water despite the improvement in crystallinity ofTiO₂(B). As a result of the excessive desorption of crystal water, thecrystallite size is increased, and an interplanar spacing is decreased,thereby reducing the reversible charge-discharge capacity. In contrast,when the peak half-width (2θ) is larger than 4 degrees, thecrystallinity is considerably deteriorated to undesirably cause adecrease in electrode capacity performance and a prominent decrease incycle life performance. Since it is possible to expose many crystalplanes in which the lithium ion desorption/insertion is easy by keepingthe half-width (2θ) to 0.5 degree or more and 4 degrees or less, it ispossible to improve diffusion of lithium ions, thereby making itpossible to increase the effective electrode capacity to a reduce adifference from the theoretical value as well as to improve thecharge-discharge cycle performance. A more preferred range of thehalf-width (2θ) may be from 0.6 degree or more to 2 degrees or less.

While improving the effective electrode capacity and charge-dischargecycle performance, the above-described titanium oxide compound iscapable of maintaining an electrode potential to about 1.5 V based on ametal lithium, i.e., to an electrode potential attained by spinel typelithium titanate. Therefore, it is possible to realize a nonaqueouselectrolyte battery and a battery pack which has a high energy densityand an excellent cycle performance, and is capable of stably conductingrapid charge-discharge.

In the X-ray powder diffraction measurement using Cu-Kα radiation, whenTiO₂(B) is oriented to the (001) plane, peak intensity at Miller indicesof which orientation is the same as the orientation of a reflectionplane of the (001) plane is high. More specifically, it is possible todetect the orientation since the peak intensities corresponding to the(001) plane which appears in the vicinity of 2θ=14.25°, the (002) planewhich appears in the vicinity of 2θ=28.68°, and the (003) plane whichappears in the vicinity of 2θ=43.63° are higher than those of otherMiller indices. In view of the fact that the (110) plane which appearsnear 2θ=25° is observed as the highest intensity peak in the titaniumdioxide described in ASTM card (No. 35-0088) which is the index of X-raypowder diffraction pattern and R. Marchand, L. Brohan, M. Tournoux,Material Research Bulletin 15, 1129 (1980) and JP-A 2008-117625 (KOKAI),the orientation is not directed toward the (001) plane.

It is desirable that the titanium oxide compound satisfies the followingexpression (1):I(110)/I(00Z)≦1  (1),

provided that I(00Z) represents intensity of a highest intensity peak,and I(110) represents a peak intensity of the (110) plane in the X-raypowder diffractometry.

The above-described performance improvement effect due to the grainsoriented in the direction of the (001) plane is prominently exhibitedwhen the peak intensity ratio of the expression (1) is kept to one orless. Further, when the complete orientation to the (001) plane isattained, the peak intensity ratio substantially becomes zero. The peakintensity ratio of zero is not observed in most cases of crystal grainssuch as polycrystals, but the intensity ratio can be zero in a thin filmof which orientation is controlled on a special substrate. Further,since it is easier to attain the effect of the (001) plane orientationwith the use of the thin film, the thin film can be one of preferredmodes.

Hereinafter, a measurement method employing X-ray powder diffractometryusing Cu-Kα radiation will be described. A sample is pulverized until anaverage particle diameter becomes about 10 μm. After that, a holderportion having a depth of 0.2 mm in a glass test plate is filled withthe sample, and the sample is flattened for a measurement bysufficiently pressing a glass plate to the sample from the outside withfingers. It is necessary to make sure that the measurement sample issufficiently filled at the holder portion, and it is necessary to payattention not to overlook filling deficiency of the sample, such as acrack and a clearance. Further, the sample should be filled to be equalto the depth (0.2 mm) of the glass holder, and it is necessary to payattention not to allow unevenness from the reference surface of theglass holder which can be caused by an excessive/insufficient fillingamount. Examples of a more preferred method include the followingmethods. In order to eliminate a shift of a diffraction peak position ora change in intensity ratio which can be caused when the sample isfilled in the glass sample plate, compacted pellets each having adiameter of 10 mm and a thickness of 2 mm are obtained by applying apressure of about 250 MPa for 15 minutes, and the surface of the pelletis measured.

Further, in the case of performing an X-ray powder diffractionmeasurement of a titanium oxide compound using Cu-Kα radiation, it ispossible to measure an electrode containing the titanium oxide compoundinstead of the measurement of the powder of the compound. For example,an electrode is produced by preparing a slurry by suspending a powder ofthe titanium oxide compound, a binder, and a conduction agent into awidely-used solvent, coating the slurry on a current collector, dryingto form an electrode layer, and pressing. It is possible to determine anorientation and a highest intensity peak by subjecting the obtainedelectrode to the X-ray diffraction measurement. More specifically,diffraction peaks attributable to the current collector and addedcomponents such as the conduction agent are eliminated from ameasurement result to extract only a diffraction peak corresponding toTiO₂(B), and an intensity relationship is investigated to detect theorientation of the (001) plane and the highest intensity peak. With themeasurement method, since the degree of orientation is increased by thepressing in the case where TiO₂(B) has orientation on a specific crystalplane, while the measurement result has no influence in the case whereTiO₂(B) does not have orientation, the measurement method is preferredin terms of the orientation detection.

It is possible to estimate a width of the peak at a positioncorresponding to an intensity value which is a half of the highestintensity peak value as a half-width (2θ) (FWHM).

Though an average particle diameter of the titanium oxide compound isnot particularly limited, it is desirable to include a crystal having anaspect ratio within a range of 1 to 50, a short axis of 0.1 μm or moreand 50 μm or less, and a long axis of 0.1 μm or more and 200 μm or less.These ratios can be changed depending on required battery properties.For example, in the case where rapid charge-discharge is required, it isdesirable since it is possible to reduce a diffusion distance of thelithium ion in the crystal by setting the aspect ratio to one and thelong and short axes to 0.1 μm. In the case where a high capacity isrequired, it is possible to intentionally increase a plane in alongitudinal direction of the crystal, i.e., the (001) plane which isthe orientation plane, in the pressed electrode by setting the aspectratio to 10 or more, the short axis to 5 μm, and the long axis to 50 μmor more and 200 μm or less, thereby making it possible to form anelectrode having many crystal planes which are advantageous for lithiumabsorption and release. It is possible to reduce a contact area betweenthe electrode and the electrolytic solution as well as to enhancecrystallinity by setting the short and long axes to 0.1 μm or more.Further, by using a long crystal axis of which is 200 μm or less, it ispossible to attain good dispersibility of the negative active materialinto the solvent, thereby improving stability of the slurry.

In the case of aspect ratio measurement of a powder, it is possible toemploy a measurement using a laser diffractometer. For example, afterobserving a shape of particles by using an electron microscope or thelike, a particle distribution is measured by using the laserdiffractometer. When it is revealed by the electron microscopeobservation that the powder is of uniform particles and has a highaspect ratio, a peak of a particle distribution corresponding to theshort axis of the particles and a peak of the particle distributioncorresponding to the long axis of the particles appear in the particledistribution detected by the laser diffractometer, and, therefore, it ispossible to consider a ratio between the peaks as an average particledistribution. In other cases, it is possible to obtain the aspect ratioby measuring lengths of short axes and long axes of particles byelectron microscope observation.

It is possible to determine the long axis of a particle as describedbelow. In observation of a section of an electrode by using an electronmicroscope, the smallest circle (hereinafter referred to as smallestcircumcircle) among circles enclosing particles (i.e., circumcircles) isdrawn. A line between two contact points and having a maximum length isthe long axis. The contact point is one between the smallestcircumcircle and an outline of the particle.

The determination of the long axis will be described with reference toFIG. 2. A circle C is the smallest circumcircle of a scale shapeparticle 101. The circle C contacts an outline of the particle 101 atpoints P1 to P3. In the case where lengths of lines obtained byconnecting point P1 to point P2, point P2 to point P3, and point P3 topoint P1 are L12, L23, and L31, respectively, the longest line is L12.Therefore, the long axis of the particle 101 shown in FIG. 2 is lineL12.

The aspect ratio can be determined as described below. After determiningthe long axis by the above-described method, a line having a maximumlength among straight lines which are perpendicular to the long axis andpartitioned by the outline of the particle is the short axis.

When the short and long axes are determined as described above, theaspect ratio is obtained by the following expression:Aspect ratio=(long axis)/(short axis).

In FIG. 2, the long axis is L12, and the short axis is L4. Therefore, itis possible to detect the aspect ratio as L12/L4. It is possible toobtain the aspect ratio by measuring the aspect ratio obtained asdescribed above on each of a plurality of points (e.g., 100 points ormore) within a field of view of an electron microscopic picture andobtaining an average of the aspect ratios.

A BET specific surface area of the titanium oxide compound maypreferably be 5 m²/g or more and 100 m²/g or less without particularlimitation thereto. Since it is possible to ensure a required contactarea with the nonaqueous electrolyte by keeping the specific surfacearea 5 m²/g or more, it is possible to enhance the battery performance.Further, since it is possible to attain good application performance ofthe slurry to be used for the electrode production as well as tosuppress reactivity between the nonaqueous electrolyte and the negativeelectrode active material by keeping the specific surface area 100 m²/gor less, it is possible to improve a cycle life performance.

For analysis of the specific surface area, a method of causing moleculeseach of which an area to be occupied by its adsorption is detected to beadsorbed on surfaces of particles at a temperature of liquid nitrogenand obtaining a specific surface area of the particles from an amount ofadsorbed molecules is employed. The most widely employed method is theBET method utilizing low temperature low humidity physical adsorption ofan inert gas, which is obtained by expanding the Langmuir theory whichis a monomolecular layer adsorption theory to multilayer adsorption andis the most famous theory as a specific surface area calculation method.The specific surface area obtained by the theory is called BET specificsurface area.

When a TiO₂(B) crystal having an orientation to the (001) plane issynthesized by increasing the baking temperature or baking time, anatasetype titanium dioxide which is more thermally stable than TiO₂(B) tendsto be generated, and many impurity phases corresponding to polymorphismof titanium oxide are contained, which are problematic. The impurityphase is a phase which is a polymorphism phase, in other words, is aphase which has an identical composition and a different crystal shapeand a phase which contains same kinds of elements but deviates from astoichiometric ratio. Further, a crystal lattice is contracted due toexcessive desorption of crystal water in TiO₂(B) to reduce the peakhalf-width (2θ) to less than 0.5 degree, resulting in deterioration oflithium ion diffusion property in a solid matter, whereby the effect bythe orientation to the (001) plane is not exhibited.

The inventors have synthesized an alkali titanate compound such aspotassium titanate (K₂Ti₄O₉), sodium titanate (Na₂Ti₃O₇), and cesiumtitanate (Cs₂Ti₅O₁₂) by using a fusion method of fusing at 900 to 1500°C. and crystallizing by cooling, a flux method of using K₂MoO₄, K₂WO₄,or the like as a flux, or a hydrothermal synthesis for synthesizing thealkali titanate compound at 100 to 500° C. and 10 to 1000 atm with usingTiO₂ and an alkali solution in which KOH or the like is dissolved. As aresult, the inventors have managed to impart anisotropy to a crystalgrowth rate of the alkali titanate compound and have succeeded insynthesizing a raw material crystal of which the (001) plane orientationis easily achieved. The obtained alkali titanate compound is subjectedto proton exchange process to synthesize titanate (H₂Ti₄O₉ or the like)which is a precursor of TiO₂(B), and then the titanate is subjected tolow-temperature and short-time heating to synthesize TiO₂(B). Theinventors have found that it is possible to control the half-width (2θ)of the highest intensity peak within a range of 0.5 degree to 4 degreeswhile attaining the (001) plane orientation by the above-describedsynthesizing method. Since it is possible to perform the synthesis underconditions which do not cause the impurity phase and lattice contractionaccording to the method, the lithium ion desorption/insertion isfacilitated by the effect of the (001) plane orientation, thereby makingit possible to provide a negative electrode active material having ahigh capacity of 240 mAh/g or more which is 70% or more of thetheoretical capacity. A commercially available reagent of the alkalititanate compound which is synthesized by a method similar to theabove-described method may be used as a starting material.

In the above-described fusion method, anisotropic crystal growth isattained by mixing a raw material oxide, a carbonate, and the like atstoichiometric ratio, fusing at a temperature equal to or higher than amelting point, and solidification by cooling. Here, it is preferable toconduct the fusing by appropriately adding a known flux material. It ispossible to control a growth rate of a target crystal plane by changingthe type of flux material.

The flux method enables to control a crystal growth axis by usingK₂MoO₄, K₂WO₄, or the like as a flux species.

The above-described method of obtaining the alkali titanate compound ofwhich a specific crystal plane is oriented through the hydrothermalsynthesis will be described. To start with, a titanium alkoxide solutionis prepared by diluting titanium alkoxide with a solvent. Titaniumtetraisopropoxide may be used as the titanium alkoxide. Ethanol or2-propanol may be used as the solvent. The titanium alkoxide is gelledthrough hydrolysis by adding a dropping of a solution (ethanolconcentration: 20 to 50 wt %) obtained by mixing pure water and ethanolto the titanium alkoxide mixed with the solvent, and then the titaniumalkoxide is heated at 400 to 600° C. to obtain titanium oxide particles.Next, the obtained titanium oxide particles are dispersed into an alkalisolution, followed by a hydrothermal treatment. The hydrothermaltreatment may be performed by a known method. As the alkali solution, asodium hydroxide solution, a lithium hydroxide solution, or a potassiumhydroxide solution may be used. The titanium oxide particles dispersedinto alkali are subjected to hydrothermal synthesis by pressurizing andheating using an autoclave pressurizing vessel, i.e., at hightemperature and high pressure (e.g., 100 to 500° C., 10 to 1000 atm).Here, since the alkali titanate compound having at least one of ananotube structure and a nanowire structure in which a specific crystalplane is grown is generated, it is possible to synthesize TiO₂(B) whichhas the (001) orientation in the same manner as in the fusion method.

In order to remove impurities from a powder of the raw material alkalititanate compound synthesized by the fusion method, flux method, andhydrothermal method, the powder is washed well with distilled water andsubjected to an acid treatment, and then alkali cations are exchangedwith protons. It is possible to perform the proton exchange of each ofpotassium ion contained in potassium titanate, sodium ion contained insodium titanate, and cesium ion contained in cesium titanate withoutbreaking the crystal structures. The proton exchange by acid treatmentis performed by adding hydrochloric acid, nitric acid, or sulfuric acidhaving a concentration of 0.5 to 2 M to the powder, followed bystirring. It is desirable to keep the acid treatment performed until theproton exchange is satisfactorily completed. It is necessary to payattention not to remain potassium, sodium, or the like contained in theraw material, since the remaining potassium or the like results in areduction of charge-discharge capacity. The acid treatment may desirablybe performed at room temperature of about 25° C. without particularlimitation thereto and for 24 hours or more in the case wherehydrochloric acid having a concentration of about 1M is used. Morepreferably, the acid treatment is performed for 1 to 2 weeks. Further,in order to perform the proton exchange without fail, it is desirable tochange the acid solution with new one every 24 hours. In order toneutralize the residual acid in the proton exchange, an alkali solutionsuch as a lithium hydroxide solution may be added. After completion ofthe proton exchange, washing well with distilled water is performedagain to make a pH of the washing water to be settled within a range of6 to 8. By washing the product after the proton exchange with water anddrying, a proton titanate compound which is an intermediate product(precursor) is obtained.

Next, a titanium oxide compound which is the final target product isobtained by subjecting the proton titanate compound which is theintermediate product (precursor) synthesized by the above-describedmethods to a heat treatment. An optimum temperature in the heattreatment of the proton titanate compound is varied depending on a rawmaterial composition, a particle diameter, a crystal shape, and the likeof the proton titanate compound. It is possible to synthesize thetitanium oxide compound having a high capacity by controlling heatingtemperature and time when any one of the raw materials is used. Theheating temperature is within a range of 300 to 500° C., and a range of350 to 400° C. is particularly preferred for maintaining the (001) planeorientation. When the heating temperature is less than 300° C.,crystallinity is considerably deteriorated, and an electrode capacity,charge-discharge efficiency, a repetition performance are undesirablydeteriorated. When the heating temperature exceeds 500° C.,rearrangement of atoms in the crystal is promoted, which not onlydeteriorates the (001) orientation of TiO₂(B), but also generatesanatase type titanium dioxide as an impurity phase, resulting inundesirable deterioration of electrode performance.

In the known synthetic methods described in R. Marchand, L. Brohan, M.Tournoux, Material Research Bulletin 15, 1129 (1980) and JP-A2008-117625 (KOKAI), since the isotropic raw material which issynthesized by a solid phase reaction is used, it is difficult to exposethe (001) plane having the tunneling structure which is advantageous forthe lithium desorption/insertion in the electrode. Therefore, it isconsidered that the electrode capacity is suppressed to 160 to 200 mAh/gor less than this, and the negative electrode active material containingthe titanium oxide compound in which more (001) planes are exposed isadvantageous for stably providing the high electrode capacity.

According to the first embodiment, it is possible to provide a negativeelectrode material, which has a high initial discharge capacity and anexcellent charge-discharge cycle performance.

Second Embodiment

A nonaqueous electrolyte battery according to the second embodimentincludes a positive electrode, a negative electrode containing thenegative electrode active material according to the first embodiment, anonaqueous electrolyte, a separator, and a jacket member.

Hereinafter, the positive electrode, the negative electrode, thenonaqueous electrolyte, the separator, and the jacket member will bedescribed in detail.

The positive electrode includes a current collector and a layer(positive electrode active material-containing layer) provided on one orboth of surfaces of the current collector and containing a positiveelectrode active material and a binder.

Examples of the positive electrode active material include an oxide, asulfide, and the like. Specific examples thereof include manganesedioxide (MnO₂) capable of absorbing lithium, iron oxide, copper oxide,nickel oxide, lithium manganese composite oxide (e.g., Li_(x)Mn₂O₄ orLi_(x)MnO₂), lithium nickel composite oxide (e.g., Li_(x)NiO₂), lithiumcobalt composite oxide (e.g., Li_(x)CoO₂), lithium nickel cobaltcomposite oxide (e.g., LiNi_(1-y)Co_(y)O₂), lithium manganese cobaltcomposite oxide (e.g., Li_(x)Mn_(y)Co_(1-y)O₂), spinel type lithiummanganese nickel composite oxide (Li_(x)Mn_(2-y)Ni_(y)O₄), lithiumphosphor oxide having an olivine structure (e.g., Li_(x)FePO₄,Li_(x)Fe_(1-y)Mn_(y)PO₄, Li_(x)CoPO₄), iron sulfate (Fe₂(SO₄)₃),vanadium oxide (e.g., V₂O₅), lithium nickel cobalt manganese compositeoxide, and the like. Here, each of x and y is within a range of 0 to 1.

Examples of the positive electrode active material which enables toobtain a high positive electrode potential include lithium manganesecomposite oxide (Li_(x)Mn₂O₄), lithium nickel composite oxide(Li_(x)NiO₂), lithium cobalt composite oxide (LixCoO₂), lithium nickelcobalt composite oxide (LiNi_(1-y)Co_(y)O₂), spinel type lithiummanganese nickel composite oxide (e.g., Li_(x)Mn_(2-y)Ni_(y)O₄), lithiummanganese cobalt composite oxide (Li_(x)Mn_(y)Co_(1-y)O₂), lithium ironphosphate (Li_(x)FePO₄), lithium nickel cobalt manganese compositeoxide, and the like. Here, each of x and y is within a range of 0 to 1.

Among the above, it is preferable to use lithium iron phosphate,Li_(x)VPO₄F, lithium manganese composite oxide, lithium nickel compositeoxide, lithium nickel cobalt composite oxide, or the like in the case ofusing a nonaqueous electrolyte containing an ionic liquid from theviewpoint of cycle life. With the use of such active material, it ispossible to reduce reactivity between the positive active material andthe ionic liquid. A primary particle diameter of the positive electrodeactive material may preferably be 100 nm or more and 1 μm or less. Thepositive electrode active material having the primary particle diameterof 100 nm or more enables easy handling in terms of industrialproduction. The positive electrode active material having the primaryparticle diameter of 1 μm or less enables to smoothly diffuse lithiumions into a solid matter.

A specific surface area of the positive electrode active material maypreferably be 0.1 m²/g or more and 10 m²/g or less. The positiveelectrode active material having the specific surface area of 0.1 m²/gor more enables to ensure sufficient absorption/desorption sites forlithium ions. The positive electrode active material having the specificsurface area of 10 m²/g or less enables easy handling in terms ofindustrial production and enables to ensure a good charge-dischargecycle performance.

Examples of the binder which is used for the purpose of binding thepositive electrode active material with the current collector includepolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), afluorine-based rubber, and the like.

A conductive agent may be added as required in order to enhance acurrent collection performance and to suppress contact resistance withthe current collector. Examples of the conductive agent include acarbonaceous material such as acetylene black, carbon black, andgraphite.

As proportions of the positive electrode active material and the binder,the positive electrode active material may preferably be within a rangeof 80 wt % or more to 98 wt % or less, and the binder may preferably bewithin a range of 2 wt % or more to 20 wt % or less. It is possible toattain sufficient electrode strength by keeping the amount of the binderto 2 wt % or more, and it is possible to reduce a content of aninsulator in the electrode and to reduce internal resistance by keepingthe amount of the binder to 20 wt % or less.

In the case of adding the conductive agent, it is possible to attain theabove-described conductive agent addition effect by keeping an amountthereof to 3 wt % or more, and it is possible to reduce decomposition ofthe nonaqueous electrolyte on surfaces of the positive electrodeconductive agent under high-temperature storage by keeping the amountthereof to 15 wt % or less.

The positive electrode is produced by preparing a slurry by suspendingthe positive electrode active material, the binder, and the conductiveagent as required into an appropriate solvent, coating the slurry on thepositive electrode current collector, drying to form the positiveelectrode active material-containing layer, and pressing, for example.

In the manufacture of the positive electrode, a positive electrodeactive material-containing layer which is produced by mixing thepositive electrode active material, the binder, and the conductive agentas required may be used, and followed by molding the mixture into apellet.

The positive electrode current collector may preferably be an aluminumfoil or an aluminum alloy foil.

A thickness of the aluminum foil or the aluminum alloy foil maypreferably be 5 μm or more and 20 μm or less, more preferably 15 μm orless. The aluminum foil may preferably have a purity of 99 wt % or more.The aluminum alloy may be an alloy containing an element such asmagnesium, zinc, silicon, and the like. A content of a transition metalwhich can be contained in the aluminum foil or the aluminum alloy foilmay preferably be 1 wt % or less. Examples of the transition metalinclude iron, copper, nickel, and chromium.

The negative electrode includes a negative electrode current collectorand a layer (negative electrode active material-containing layer)provided on one or both of surfaces of the current collector andcontaining a negative electrode active material, a conductive agent, anda binder. In the layer, the binder fills clearances of the dispersednegative electrode active material, and the conductive agent is addedfor the purpose of enhancing a current collector performance andsuppressing contact resistance with the current collector.

Examples of the negative electrode active material include the titaniumoxide compound used in the first embodiment.

The titanium oxide compound may be used alone as the negative electrodeactive material or may be used in combination with other negativeelectrode active materials. Preferred examples of the other negativeelectrode active materials include anatase type titanium dioxide (TiO₂),Li₂Ti₃O₇ which is ramsdellite type lithium titanate, Li₄Ti₅O₁₂ which isspinel type lithium titanate, since they have a similar specific weightand are well mixed and dispersed.

A content of the negative electrode active material in the layer may be70 wt % or more and 98 wt % or less.

Examples of the conductive agent include a carbonaceous material such asacetylene black, carbon black, graphite, carbon nanotube, and carbonnanofiber.

Examples of the binder include polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), a fluorine-based rubber, astyrene-butadiene rubber, and the like.

The binder may preferably be mixed in the layer in an amount of 2 wt %or more and 30 wt % or less. When the amount of the binder is 2 wt % ormore, an excellent cycle performance is expected since satisfactorybinding between the layer and the current collector is attained. Incontrast, from the viewpoint of high capacity, the amount of the bindermay preferably be 30 wt % or less. Further, a proportion of theconductive agent in the layer may preferably be 30 wt % or less.

For the current collector, a material which is electrochemically stableat a lithium absorption/release potential of the negative electrodeactive material is used. The current collector may preferably be madefrom copper, nickel, stainless steel, or aluminum. A thickness of thecurrent collector may preferably be 5 to 20 μm. The current collectorhaving the above-specified thickness is capable of keeping a balancebetween strength and light weight of the negative electrode.

The negative electrode is produced by preparing a slurry by suspendingthe negative electrode active material, the conductive agent, and thebinder into a widely-used solvent, coating the slurry on the currentcollector, drying to form the layer, and pressing, for example.

In the manufacture of the negative electrode, a negative electrodeactive material-containing layer which is produced by mixing thenegative electrode active material, the binder, and the conductive agentmay be used, and followed by molding the mixture into a pellet.

Examples of the nonaqueous electrolyte include a liquid nonaqueouselectrolyte which is prepared by dissolving an electrolyte into anorganic solvent, a gel nonaqueous electrolyte which is a composite of aliquid electrolyte and a polymer material, and the like.

The liquid nonaqueous electrolyte may be prepared by dissolving theelectrolyte into the organic solvent at a concentration of 0.5 mol/L ormore and 2.5 mol/L or less.

Examples of the electrolyte include lithium salts such as lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄), lithium hexafluoro arsenate (LiAsF₆), lithiumtrifluoromethasulfonate (LiCF₃SO₃), and lithium bis(trifluoromethylsulfonyl)imide [LiN(CF₃SO₂)₂] or mixtures thereof. The electrolyte maypreferably be hardly oxidized at a high potential, and LiPF₆ is mostpreferred.

Examples of the organic solvent include a cyclic carbonate such aspropylene carbonate (PC), ethylene carbonate (EC), and vinylenecarbonate; a chain carbonate such as diethyl carbonate (DEC), dimethylcarbonate (DMC), and methylethyl carbonate (MEC); a cyclic ether such astetrahydrofuran (THF), 2-methyl tetrahydrofuran (2MeTHF), and dioxolan(DOX); a chain ether such as dimethoxyethane (DME) and diethoxy ethane(DEE); γ-butyrolactone (GBL); acetonitrile (AN); sulfolane (SL); and thelike, which may be used alone or in the form of a mixture solvent.

Examples of the polymer material include polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), polyethylene oxide (PEO), and the like.

As the nonaqueous electrolyte, an ionic liquid containing lithium ion, apolymeric solid electrolyte, an inorganic solid electrolyte, or the likemay be used.

The ionic liquid means a compound which can exist as a liquid at anambient temperature (15 to 25° C.) among organic salts obtained bycombining an organic cation and an anion. Examples of the ionic liquidinclude those existing as a liquid when used alone, those becoming aliquid when mixed with an electrolyte, those becoming a liquid whendissolved into an organic solvent, and the like. A melting point of theionic liquid which is typically used for the nonaqueous electrolytebattery is 25° C. or less. Further, the organic cation ordinarily has aquaternary ammonium skeleton.

The polymeric solid electrolyte is prepared by dissolving theelectrolyte into the polymer material, followed by solidification.

The inorganic solid electrolyte is a solid substance having lithium ionconductivity.

Examples of the separator include a porous film containing polyethylene,polypropylene, cellulose, or polyvinylidene fluoride (PVdF), a nonwovenfabric of a synthetic resin, and the like. Among the above, the porousfilm containing polyethylene or polypropylene is preferred from theviewpoint of improvement in safety since such a porous film is molten ata certain temperature and blocks a current.

For the jacket member, a laminate film having a thickness of 0.5 mm orless or a metallic container having a thickness of 1 mm or less is used.The thickness of the laminate film may more preferably be 0.2 mm orless. The metallic container may more preferably have a thickness of 0.5mm or less, further preferably 0.2 mm or less.

Examples of a shape of the jacket member include a flat type (thintype), a square type, a cylinder type, a coin type, a button type, andthe like. Depending on battery dimensions, a jacket member for smallbattery which is mounted to a mobile electronic appliance, a jacketmember for a large battery which is mounted on a two- or four-wheelvehicle, and the like may be used.

As the laminate film, a multilayer film in which a metal layer is formedbetween resin layers is used. The metal layer may preferably be analuminum foil or an aluminum alloy foil for attaining a light weight. Asthe resin layer, a polymer material such as polypropylene (PP),polyethylene (PE), nylon, polyethylene terephthalate (PET), and the likemay be used. The laminate film may be molded into the shape of thejacket member by sealing by thermal fusion bonding.

The metallic container is made from aluminum, an aluminum alloy, or thelike. The aluminum alloy may preferably be an alloy containing anelement such as magnesium, zinc, silicon, and the like. A content of atransition metal such as iron, copper, nickel, and chromium in thealuminum or aluminum alloy may preferably be 1 wt % or less. With suchcontent, it is possible to dramatically improve a long-term reliabilityand a heat discharge performance in a high-temperature environment.

Hereinafter, the nonaqueous electrolyte battery according to the secondembodiment will be more specifically described with reference to FIG. 3and FIG. 4. FIG. 3 is a sectional view showing the flat nonaqueouselectrolyte battery according to the second embodiment, and FIG. 4 is anenlarged view of a part A of FIG. 3.

A flat wound electrode group 1 is housed in a sac-like jacket member 2made from a laminate film which is obtained by inserting a metal layerinto two resin layers. The flat wound electrode group 1 is formed byspirally winding a laminate obtained by laminating a negative electrode3, a separator 4, a positive electrode 5, and a separator 4 in thisorder from an outermost layer and press-molding. The negative electrode3 which serves as the outermost layer has a structure that a layer 3 bcontaining the above-described negative electrode active materialaccording to the first embodiment is formed on an inner surface of anegative electrode current collector 3 a as shown in FIG. 4, and therest of the negative electrodes 3 have a structure that the layer 3 b isformed on both surfaces of the negative electrode current collector 3 a.The positive electrode 5 has a structure that a layer 5 b is formed onboth surfaces of a positive electrode current collector 5 a.

In the vicinity of an outer periphery of the wound electrode group 1, anegative electrode terminal 6 is electrically connected to the negativeelectrode current collector 3 a of the negative electrode 3 serving asthe outermost layer, and a positive electrode terminal 7 is electricallyconnected to the positive electrode current collector 5 a of the innerpositive electrode 5. The negative electrode terminal 6 and the positiveelectrode terminal 7 are extended from an opening of the sac-like jacketmember 2 to the outside. For example, the liquid nonaqueous electrolyteis injected through the opening of the sac-like jacket member 2. Thewound electrode group 1 and the liquid nonaqueous electrolyte arecompletely sealed by heat-sealing the opening of the sac-like jacketmember 2 with the negative electrode terminal 6 and the positiveelectrode terminal 7 being inserted into the opening.

The negative electrode terminal may be formed from a material which haselectrochemical stability and conductivity at the above-described Liabsorption/release potential of the negative electrode active material.Specific examples thereof include copper, nickel, stainless steel, andaluminum. The negative electrode terminal may preferably be a materialwhich is the same as that used for the negative electrode currentcollector in order to reduce contact resistance.

The positive electrode terminal may be formed from a material which haselectrical stability and conductivity when a potential to a lithium ionmetal is within a range of 3 to 5 V. Specific examples thereof includealuminum and an aluminum alloy containing an element such as Mg, Ti, Zn,Mn, Fe, Cu, and Si. The positive electrode terminal may preferably be amaterial which is the same as that used for the positive electrodecurrent collector in order to reduce contact resistance.

The nonaqueous electrolyte battery according to the second embodimentmay have the structure shown in FIG. 5 and FIG. 6 without limitation tothe above-described structure shown in FIG. 3 and FIG. 4. FIG. 5 is anoblique view, partly broken away, schematically showing anothernonaqueous electrolyte battery according to the second embodiment, andFIG. 6 is an enlarged sectional view showing a part B of FIG. 5.

A laminate type electrode group 11 is housed in a jacket member 12 madefrom a laminate film in which a metal layer is formed between two resinlayers. The laminate type electrode group 11 has a structure that apositive electrode 13 and a negative electrode 14 are alternatelylaminated with a separator 15 being disposed therebetween as shown inFIG. 6. There are a plurality of the positive electrodes 13, and each ofwhich includes a current collector 13 a and a positive electrode activematerial-containing layer 13 b which is carried on both surfaces of thecurrent collector 13 a. There are a plurality of the negative electrodes14, and each of which includes a current collector 14 a and a negativeelectrode active material-containing layer 14 b which is carried on bothsurfaces of the current collector 14 a. One side of the currentcollector 14 a of each of the negative electrode 14 is projected fromthe positive electrode 13. The projected current collector 14 a iselectrically connected to a strip-like negative electrode terminal 16. Aleading end of the strip-like negative electrode terminal 16 is drawn tothe outside from a jacket member 11. Further, though not shown, as forthe current collector 13 a of the positive electrode 13, a sidepositioned at an opposite side of the projected side of the currentcollector 14 a is projected from the negative electrode 14. The currentcollector 13 a projected from the negative electrode 14 is electricallyconnected to a strip-like positive electrode terminal 17. A leading endof the strip-like positive electrode terminal 17 is positioned at anopposite side of the negative electrode terminal 16 and is drawn to theoutside from one side of the jacket member 11.

According to the second embodiment as described above, it is possible toprovide the nonaqueous electrolyte battery having the high initialdischarge capacity and the excellent repetitive charge-dischargeperformance owing to the negative electrode containing the negativeelectrode active material described in the first embodiment.

Third Embodiment

The battery pack according to the third embodiment has a plurality ofunit cells, in which the unit cells are connected with each other inelectrically series or parallel. It is possible that the nonaqueouselectrolyte battery is used as the unit cell.

The battery pack will be described in detail with reference to FIG. 7and FIG. 8. As the cell, the flat battery shown in FIG. 3 is usable.

A plurality of the unit cells 21 each formed of the flat nonaqueouselectrolyte battery shown in FIG. 3 are stacked so that a positiveelectrode terminal 7 and a negative electrode terminal 6 that areextended to outside are aligned in the same direction, and are bound byan adhesive tape 22 to constitute a battery module 23. As shown in FIG.8, the unit cells 21 are connected electrically in series with eachother.

A printed wiring board 24 is disposed opposing to the side surface ofthe unit cells 21 from which the negative electrode terminal 6 andpositive electrode terminal 7 are extended. As shown in FIG. 8, athermistor 25, a protective circuit 26, and a terminal 27 for carrying acurrent to an external device are mounted on the printed wiring board24. In addition, an insulating board (not shown) is attached to thesurface of the protective circuit substrate 24, which faces the batterymodule 23, so as to avoid unnecessary connection with the wiring of thebattery module 23.

A positive electrode lead 28 is electrically connected to the positiveelectrode terminal 7 that is positioned at the lowermost layer of thebattery module 23, and the tip thereof is inserted to and electricallyconnected to a positive electrode connector 29 of the printed wiringboard 24. A negative electrode lead 30 is electrically connected to thenegative electrode terminal 6 that is positioned at the uppermost layerof the battery module 23, and the tip thereof is inserted to andelectrically connected to a negative electrode connector 31 of theprinted wiring board 24. These connectors 29 and 31 are connected to aprotective circuit 26 via wirings 32 and 33 that are formed on theprinted wiring board 24.

The thermistor 25 detects the temperature of the unit cells 21, and thedetection signal thereof is sent to the protective circuit 26. Theprotective circuit 26 may break a plus wiring 34 a and a minus wiring 34b between the protective circuit 26 and the terminal 27 for carrying acurrent to an external device, under a predetermined condition. Thepredetermined condition refers to, for example, the time at which thedetection temperature of the thermistor 25 reaches a predeterminedtemperature or more. Furthermore, the predetermined condition refers tothe time at which over-charge, over-discharge, over-current or the likeof the unit cells 21 are detected. The detection of over-charge or thelike is performed in the individual unit cells 21 or the entirety of theunit cells 21. When detection is performed in the individual unit cell21, a battery voltage may be detected, or a positive electrode potentialor negative electrode potential may be detected. In the latter case, alithium electrode that is used as a reference electrode is inserted inthe individual unit cell 21. In the case of FIGS. 7 and 8, wirings 35for detection of a voltage are connected to the respective unit cells21, and detection signals are sent to the protective circuit 26 via thewirings 35.

Protective sheets 36 made of a rubber or resin are disposed respectivelyon the three side surfaces of the battery module 23 except for the sidesurface from which the positive electrode terminal 7 and negativeelectrode terminal 6 protrude.

The battery module 23 is housed in a housing container 37 together withthe respective protective sheets 36 and the printed wiring board 24.Namely, the protective sheets 36 are disposed respectively on the bothinner surfaces in the longitudinal side direction and the inner surfacein the short side direction of the housing container 37, and the printedwiring board 24 is disposed on the inner surface on the opposite side inthe short side direction. The battery module 23 is positioned in a spacesurrounded by the protective sheets 36 and the printed wiring board 24.A lid 38 is attached to the upper surface of the housing container 37.

Alternatively, the battery module 23 may be fixed by using a heat shrinktape instead of the adhesive tape 22. In this case, the protectivesheets are disposed on both side surfaces of the battery module, thebattery module is wound around a heat shrink tube, and the heat shrinktube is shrank by heating to bind the battery module.

Although an embodiment in which the unit cells 21 are connected witheach other in series is shown in FIGS. 7 and 8, the unit cells may beconnected with each other in parallel so as to increase a batterycapacity. Alternatively, assembled battery packs may be connected witheach other in series or parallel.

Furthermore, the embodiment of the battery pack is suitably changedaccording to use. Preferable use of the battery pack is one for whichcycle performance at high currents is desired. Specific examples mayinclude uses in power sources for digital cameras, and in-car uses intwo to four-wheeled hybrid battery automobiles, two to four-wheeledbattery automobiles, motor assisted bicycles and the like. The use forcar is particularly suitable.

EXAMPLES

Hereinafter, a more detailed description will be given based onexamples, but the embodiments are not limited only to the examples.Identification of a crystal phase and estimation of a crystal structureobtained by a reaction are conducted by employing an X-ray powderdiffractometry using Cu-Kα radiation, and a specific surface area wasmeasured by the BET method mentioned in the first embodiment. Further, acomposition of a product was analyzed by employing the ICP method toconfirm that a target material was obtained.

Example 1

As one example of an alkali titanate compound to be used as a rawmaterial, potassium titanate represented by K₂Ti₄O₉ was synthesized. Thesynthetic method may preferably be, but is not particularly limited to,a fusing method in order to accelerate crystal growth on a specificplane. As raw materials, potassium carbonate and titanium dioxide weremixed at a composition ratio (an atomic molar ratio) (K:Ti) of 1:2. Themixture was thrown into a platinum furnace and fused by heating to 1000°C.

Next, the fused mixture was taken out of the furnace and quenched bycasting onto a cooling plate, thereby accelerating a crystal growthhaving anisotropy by taking advantage of a difference in crystal growthrate. The obtained solid matter was washed with water to dissolveaggregates while eliminating a part of potassium ions. The obtainedK₂Ti₄O₉ powder had a crystal size of 20 to 100 μm in the long-axisdirection and 1 to 10 μm in the short-axis direction, which was aplate-like crystal. The plate-like crystal maintaining the crystallineform was added to a 1M hydrochloric acid solution and stirred at 25° C.for two weeks. The 1M hydrochloric acid was replaced by new one every 24hours. Since the obtained suspension had good dispersibility, separationby filtration was difficult. Therefore, separation from the solventcomponent was conducted by using a centrifugal separator. The obtainedproton exchanged H₂Ti₄O₉ powder was washed with pure water until a pH ofthe washing reached 6 to 7.

The obtained intermediate product (precursor) H₂Ti₄O₉ was heated underthree different conditions of the temperature at 350° C. for one hour(Example 1A), three hours (Example 1B), and six hours (Example 1C). Inorder to obtain an accurate heat history, each of the samples was placedin an electric furnace which was kept at the predetermined temperatureand then quickly taken out of the furnace after being heated, then wasquenched in air. The sample was dried in vacuum at 80° C. for 12 hours.

Shown in FIG. 9 is a pattern of X-ray powder diffraction of the powderobtained by Example 1B detected by using Cu-Kα as a radiation source. Inthe X-ray powder diffraction patterns of FIG. 9 onward, the horizontalaxis represents 20, and the vertical axis represents intensity. InExamples, for conducting the X ray diffraction measurement, compactedpellets each having a diameter of 10 mm and a thickness of 2 mm wereprepared by pulverizing until an average particle diameter reached about10 μm and then pressurizing with 250 MPa for 15 minutes, and a surfaceof the pellet was measured. Measurement conditions were: scanning speedof 3 degree/minute; a step width of 0.2 degree; a tube voltage of 40 kV;and a tube current of 20 mA. As shown in FIG. 9, it was confirmed thateach of the obtained diffraction patterns was of titanium dioxidebelonging to a space group of C2/m and having the monoclinic TiO₂(B)structure. Further, in each of the diffraction patterns, it wasconfirmed that a (001) plane exhibited the highest peak intensity, and ahalf-width (28) thereof was 1.70 degrees.

Example 2

A K₂Ti₄O₉ powder in the obtained crystalline form, which was obtained inthe same manner as in Example 1, was added to a 2M hydrochloric acidsolution, then stirred at 70° C. for 48 hours. Since the obtainedsuspension had good dispersibility, separation by filtration wasdifficult. Therefore, separation from the solvent component wasconducted by using a centrifugal separator. The obtained protonexchanged H₂Ti₄O₉ powder was washed with pure water until a pH of thewashing reached 6 to 7.

The obtained intermediate product (precursor) H₂Ti₄O₉ was heated underthree different conditions of the temperature at 350° C. for one hour(Example 2A), three hours (Example 2B), and six hours (Example 2C). Inorder to obtain an accurate heat history, each of the samples was placedin an electric furnace which was kept at the predetermined temperatureand then quickly taken out of the furnace after being heated, thenquenched in air. The sample was dried in vacuum at 80° C. for 12 hours.Shown in FIG. 10 is a pattern of X-ray powder diffraction of the powderobtained by Example 2B detected by using Cu-Kα as a radiation source. Itwas confirmed that each of the obtained diffraction patterns was oftitanium dioxide belonging to a space group of C2/m and having themonoclinic TiO₂(B) structure. Further, in each of the diffractionpatterns, it was confirmed that a (002) plane exhibited the highest peakintensity, and a half-width (20) thereof was 1.86 degrees.

Example 3

A K₂Ti₄O₉ powder which was obtained in the same manner as in Example 1was pulverized by ball mill until a size in the long-axis direction of 5to 50 μm and in the short-axis direction of about 1 to 5 μm was attainedand then added to a 1M hydrochloric acid solution as in Example 1, thenstirred at 25° C. for two weeks. Since the obtained suspension had gooddispersibility, separation by filtration was difficult. Therefore,separation from the solvent component was conducted by using acentrifugal separator. The obtained proton exchanged H₂Ti₄O₉ powder waswashed with pure water until a pH of the washing reached 6 to 7.

The obtained intermediate product (precursor) H₂Ti₄O₉ was heated underthree different conditions of the temperature at 350° C. for one hour(Example 3A), three hours (Example 3B), and six hours (Example 3C). Inorder to obtain an accurate heat history, each of the samples was placedin an electric furnace which was kept at the predetermined temperatureand then quickly taken out of the furnace after being heated, thenquenched in air. The sample was dried in vacuum at 80° C. for 12 hours.Shown in FIG. 11 is a pattern of X-ray powder diffraction of the powderobtained by Example 3B detected by using Cu-Kα as a radiation source. Itwas confirmed that each of the obtained diffraction patterns was oftitanium dioxide belonging to a space group of C2/m and having themonoclinic TiO₂(B) structure. Further, in each of the diffractionpatterns, it was confirmed that a (003) plane exhibited the highest peakintensity, and a half-width (20) thereof was 1.10 degrees.

Example 4

A proton titanate compound which is a precursor of TiO₂(B) wassynthesized by a hydrothermal method. Commercially available particulatetitanium dioxide (average particle diameter: 100 nm) was used as astarting material. To a mixture solution of 60 mL of a sodium hydroxidesolution of which a concentration was adjusted to 10 mol/L and 60 mL ofethanol, 1 g of the particulate titanium dioxide was added and thenthoroughly stirred and dispersed. The dispersion liquid was transferredto a 150 mL stainless steel autoclave pressure vessel having apolytetrafluoroethylene inner wall and heated at 180° C. for 24 hours.Thus, a nanotube compound in which a specific crystal plane was grownwas obtained. After being cooled to a room temperature, the product waswashed with 0.5 M hydrochloric acid, and washed with water. The productwas placed in a reduced pressure drier to be dried at 80° C. for 12hours, thereby obtaining a nanotube proton titanate compound. Afterthat, titanium dioxide having the TiO₂(B) structure was obtained byheating the compound at 350° C. for 3 hours. The X-ray powderdiffraction measurement results of the material are shown in FIG. 12.The measurement method and measurement conditions of the X-ray powderdiffractometry were the same as those of Example 1. Likewise, from theobtained result, it was confirmed that each of the obtained diffractionpatterns was of titanium dioxide belonging to a space group of C2/m andhaving the monoclinic TiO₂(B) structure. Further, in each of thediffraction patterns, it was confirmed that a (001) plane exhibited thehighest peak intensity, and a half-width (2θ) thereof was 2.45 degrees.

Example 5

An intermediate product (precursor) H₂Ti₄O₉ obtained in the same manneras in Example 1 was heated at 350° C. for 3 hours to obtain titaniumdioxide having the target monoclinic TiO₂(B) crystal structure. Theobtained product was placed in a zirconia pot having a capacity of 100cm³, and zirconia balls each having a diameter of 10 mm were added tooccupy ⅓ of the pot capacity. The pot was rotated at 800 rpm for a ballmill treatment time of one hour (Example 5A) and 3 hours (Example 5B) sothat aspect ratios of 1.6 and 1.2 were attained, respectively. Afterthat, an X-ray powder diffraction measurement of each of the samples wasconducted. Measurement conditions were the same as those of Example 1.Though it was confirmed that the obtained diffraction patterns were oftitanium dioxide belonging to a space group of C2/m and having themonoclinic TiO₂(B) structure, peak intensity ratios I(110)/I(001) eachbetween a (001) plane having the highest peak and a (110) plane were0.54 and 0.98, respectively.

Comparative Example 1

A K₂Ti₄O₉ powder in the obtained crystalline form, which was obtained inthe same manner as in Example 1, was added to a 1M hydrochloric acidsolution, and stirred at 25° C. for 2 weeks. Since the obtainedsuspension had good dispersibility, separation by filtration wasdifficult. Therefore, separation from the solvent component wasconducted by using a centrifugal separator. The obtained protonexchanged H₂Ti₄O₉ powder was washed with pure water until a pH of thewashing reached 6 to 7.

The obtained intermediate product (precursor) H₂Ti₄O₉ was heated at 400°C. for 20 hours. In order to obtain an accurate heat history, the samplewas placed in an electric furnace which was kept at the predeterminedtemperature and then quickly taken out of the furnace after beingheated, then quenched in air. The sample was dried in vacuum at 80° C.for 12 hours. Shown in FIG. 13 is a pattern of X-ray powder diffractionof the powder obtained by Comparative Example 1 detected by using Cu-Kαas a radiation source. It was confirmed that the obtained diffractionpattern included the monoclinic TiO₂(B) structure belonging to a spacegroup of C2/m and a trace amount of an impurity phase. Further, it wasconfirmed that a (001) plane exhibited the highest peak intensity, and ahalf-width (2θ) thereof was 0.38 degree. which is narrower as comparedto the synthetic examples of Examples.

Comparative Example 2

An intermediate product (precursor) H₂Ti₄O₉ obtained in the same manneras in Comparative Example 1 was heated at 300° C. for one hour. In orderto obtain an accurate heat history, the sample was placed in an electricfurnace which was kept at the predetermined temperature and then quicklytaken out of the furnace after being heated, and quenched in air. Thesample was dried in vacuum at 80° C. for 12 hours. It was confirmed thatthe diffraction pattern detected from an X-ray powder diffractionpattern using Cu-Kα as a radiation source was of the monoclinic TiO₂(B)belonging to a space group of C2/m. Further, it was confirmed that a(001) plane exhibited the highest peak intensity, and a half-width (2θ)thereof was 4.35 degrees which is wider as compared to the syntheticexamples of Examples.

Comparative Example 3

As Comparative Example 3, a titanium oxide TiO₂—B having a bronze typestructure was synthesized by employing the synthetic method described inJP-A 2008-34368 (KOKAI). Potassium nitrate and anatase type titaniumdioxide were mixed at a predetermined ratio, and K₂Ti₄O₉ was obtained bya solid phase reaction of heating to 1000° C. for 24 hours. The compoundwas thrown into a 1M nitric acid solution, and stirred at an ambienttemperature for 12 hours. The obtained powder was washed with distilledwater several times and then heated at 400° C. for 3 hours. As shown inFIG. 14, from an X-ray powder diffraction pattern of the obtainedpowder, which was obtained by using Cu-Kα radiation source, adiffraction pattern of which a main peak was a (110) peak as in the ASTMcard No. 35-0088 was obtained, and no specific orientation was observed.

Comparative Example 4

As Comparative Example 4, titanium dioxide having bronze titanate typecrystal structure was synthesized by employing the synthetic methoddescribed in JP-A 2008-117625 (KOKAI). A sodium carbonate powder whichis a high-purity reagent material and a titanium dioxide powder wereweighed and mixed at a molar ratio Na:Ti of 2:3, and heated at 800° C.for 20 hours twice. The obtained Na₂Ti₃O₇ polycrystal was impregnatedinto a 0.5M hydrochloric acid solution and retained therein at a roomtemperature condition for 5 days, thereby performing a proton exchangetreatment. After that, a proton exchanged H₂Ti₃O₇ polycrystal which wasa precursor was obtained by washing and drying in vacuum at 120° C. for24 hours. Next, the obtained precursor H₂Ti₃O₇ polycrystal was treatedin air at 320° C. for 20 hours to obtain the titanium dioxide havingbronze titanate type crystal structure described in JP-A 2008-117625(KOKAI). From an X-ray powder diffraction pattern of the obtainedpowder, which was obtained by using Cu-Kα radiation source, adiffraction pattern of which a main peak was a (110) peak as in the ASTMcard No. 35-0088 was obtained, and no specific orientation was observed.

An electrode was obtained by mixing each of the negative electrodeactive material powders obtained by Examples and Comparative Exampleswith 10 wt % of polytetrafluoroethylene in terms of a weight ratio as abinder. In the electrode of Comparative Example 1, 30 wt % of acetyleneblack in terms of a weight ratio was added as a conductive agent. As acounter electrode of the each of the electrode, a metal lithium foil wasused. Since the lithium metal was used for the counter electrode in eachof the measurement cells, an electrode potential of Examples andComparative Examples becomes noble as compared to the counter electrode.Therefore, directions of charge-discharge are reverse to those observedwhen each of the electrodes of Examples and Comparative Examples is usedas the negative electrode. To be more specific, a reaction in whichlithium ions are inserted into the electrode of each of Examples andComparative Examples corresponds to a discharge reaction. In the presentExample, in order to avoid confusion, a reaction in which lithium ionsare inserted into the electrode of each of Examples and ComparativeExamples is referred to as charging, and a direction in which lithiumions are released is referred to as discharge. As an electrolyticsolution, a solution obtained by dissolving lithium perchlorate into amixture solvent of ethylene carbonate and diethyl carbonate (volumeratio: 1:1) at a concentration of 1M was used.

Electrochemical measurement cells were obtained by using the electrode,the counter electrode, and the electrolytic solution of Examples andComparative Examples described above. Though the electrode of each ofExamples is caused to function as a positive electrode since the lithiummetal is used for the negative electrode in the present Examples, it ispossible to cause the electrode of each of Examples to function as thenegative electrode by combining with a known positive electrodematerial.

Charge-discharge curves of Examples and Comparative Examples wereevaluated. Charge-discharge was performed within a potential range of1.0 to 3.0 V based on the metal lithium electrode reference. Thecharge-discharge test was conducted at a charge-discharge current of0.05 mA/cm² at a room temperature.

Next, in order to confirm that the negative electrode active materialsof Examples are capable of stable charging-discharging, charge-dischargeof each of the cells of Examples and Comparative Examples was performedfor 50 cycle repeatedly (one cycle consists of charge and discharge),and a discharge capacity retention ratio after 50 cycles wasinvestigated. The charge-discharges were conducted within a potentialrange of 1.0 to 3.0 V based on the metal lithium electrode reference ata charge-discharge current of 0.05 mA/cm² at a room temperature. Thedischarge capacity retention ratio after 50 cycles was calculated bysetting the initial discharge capacity at 0.05 mA/cm² as 100%.

Discharge curves of Examples 1A and 4 and Comparative Example 3 and 4are shown in FIG. 15. In Table 1, Miller indices of highest peak,intensity ratio I(110)/I(00Z), half-width (2θ) of highest peak, initialdischarge capacity, discharge capacity retention ratio after 50 cycles,initial charge-discharge efficiency, aspect ratio, and BET specificsurface area of each of Examples 1 to 5 and Comparative Examples 1 to 4are shown. The aspect ratio was calculated by measuring the short andlong axes of a particle according to the above-described microscopicobservation.

TABLE 1 Discharge Miller Half-width Initial capacity Initial indices ofof highest Intensity discharge retention charge- Specific Synthetichighest peak ratio capacity ratio after discharge Aspect surface methodpeak (degree) I(110)/I(00Z) (mAh/g) 50 cycles (%) efficiency (%) ratioarea (m²/g) Example 1A (001) 2.73 0.21 252 96.3 86.5 3.5 17.8 Example 1B(001) 1.70 0.33 235 95.1 85.5 4.3 22.5 Example 1C (001) 1.05 0.34 22893.3 87.1 7.8 18.4 Example 2A (002) 2.51 0.28 220 94.8 83.6 3.2 33.8Example 2B (002) 1.86 0.12 215 95.5 85.6 5.5 23.1 Example 2C (002) 1.010.20 218 97.2 84.6 6.7 15.5 Example 3A (003) 1.71 0.31 223 94.6 85.4 4.127.7 Example 3B (003) 1.10 0.35 211 93.8 83.8 3.8 19.8 Example 3C (003)0.88 0.29 209 94.4 84.9 8.5 16.5 Example 4 (001) 2.45 0.37 232 87.5 79.337.5 220.1 Example 5A (001) 2.21 0.54 221 92.1 84.1 1.6 26.5 Example 5B(001) 2.13 0.98 212 89.2 82.5 1.2 38.2 Comparative (001) 0.38 0.52 15895.5 85.8 3.8 7.8 Example 1 Comparative (001) 4.35 0.25 108 67.5 63.23.5 36.6 Example 2 Comparative (110) 0.27 3.06 195 80.9 85.2 — 8.4Example 3 Comparative (110) 0.31 1.10 161 78.5 83.1 — 141.0 Example 4

As is apparent from Table 1, it is confirmed that the initial dischargecapacity obtained in Examples 1 to 5 is higher than that in ComparativeExamples 1 to 4 by 20% to 50%. Further, the good results were obtainedin the charge-discharge cycle performance. Further, from the resultsshown in FIG. 15, it is understood that the discharge curve rises whenthe capacity exceeds 200 mAh/g in Examples 1A and 4, while the dischargecurve rises before the capacity reaches 200 mAh/g in ComparativeExamples 3 and 4 of which the highest peak is the (110) peak. From theseresults, the negative electrode active materials of Examples have thelarger discharge capacity and better charge-discharge cycle life ascompared to active materials synthesized by the known synthetic method.

Further, as a result of comparison among Examples 1 to 3, it isunderstood that the high initial discharge capacity is obtained inExample 1 of which the highest peak is a peak of the (001) plane.

In the Miller indices of the highest peaks described in the Examples,the peak intensity order can be changed among the (001), (002), and(003) planes depending on measurement condition such as various slits ofan X-ray powder diffraction measurement apparatus, a sampling method,and the like. Such possibility is attributed to a possibility of afluctuation in intensity of around 10% which can be caused depending ona measurement method, because the peak intensities of the (001), (002),and (003) planes are close each other. However, it is possible to attainthe effects described in the embodiments when at least one of the threeMiller indices has the highest intensity peak.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A negative electrode active material, comprising a compound having acrystal structure of monoclinic titanium dioxide, of which a highestintensity peak detected by an X-ray powder diffractometry using a Cu-Kαradiation source is a peak of a (001) plane, (002) plane, or (003)plane; and a half-width (20) of the highest intensity peak falls withina range of 0.5 degree to 4 degrees.
 2. The active material according toclaim 1, wherein the compound satisfies the following expression (1):0≦I(110)/I(00Z)≦1  (1), provided that I(00Z) is intensity of the highestintensity peak; and I(110) is intensity of a peak of a (110) plane inthe X-ray powder diffractometry.
 3. The active material according toclaim 2, wherein the compound comprises a crystal having a short axis of0.1 to 50 μm, a long axis of 0.1 to 200 μm, and an aspect ratio of 1 to50.
 4. The active material according to claim 2, wherein the compoundcomprises a crystal having the long axis of 50 to 200 μm and the aspectratio of 10 to
 50. 5. The active material according to claim 3, whereinthe compound is represented by Li_(x)TiO₂ (0≦x≦1).
 6. The activematerial according to claim 1, wherein the crystal structure ofmonoclinic titanium dioxide is represented by a space group C2/m.
 7. Theactive material according to claim 1, wherein the half-width (2θ) of thehighest intensity peak falls within a range of 0.6 degree to 2 degrees.8. The active material according to claim 1, wherein the highestintensity peak is the peak of the (001) plane.
 9. The active materialaccording to claim 1, which comprises the compound having the crystalstructure of monoclinic titanium dioxide, and at least one selected fromthe group consisting of anatase type titanium dioxide, ramsdellite typelithium titanate and spinel type lithium titanate.
 10. A nonaqueouselectrolyte battery, comprising: a positive electrode capable ofabsorbing and releasing lithium; a negative electrode comprising thenegative electrode active material defined in claim 1; and a nonaqueouselectrolyte.
 11. A battery pack, comprising a nonaqueous electrolytebattery comprising: a positive electrode capable of absorbing andreleasing lithium; a negative electrode comprising the negativeelectrode active material defined in claim 1; and a nonaqueouselectrolyte.